Add observers to FrameSinkManagerHost.

content/browser/BUILD.gn

Index: content/browser/BUILD.gn
diff --git a/content/browser/BUILD.gn b/content/browser/BUILD.gn
index b626316477b1e3356d7006a3a18a45c4d8b057ed..66511eade11c6a4b06553551937510b9ac5f1537 100644
--- a/content/browser/BUILD.gn
+++ b/content/browser/BUILD.gn
@@ -471,6 +471,7 @@ source_set("browser") {
     # needed on all platforms.
     "compositor/frame_sink_manager_host.cc",
     "compositor/frame_sink_manager_host.h",
+    "compositor/frame_sink_observer.h",
     "compositor/surface_utils.cc",
     "compositor/surface_utils.h",
     "devtools/browser_devtools_agent_host.cc",
@@ -1204,8 +1205,6 @@ source_set("browser") {
     "renderer_host/media/video_capture_provider.h",
     "renderer_host/native_web_keyboard_event_aura.cc",
     "renderer_host/native_web_keyboard_event_mac.mm",
-    "renderer_host/offscreen_canvas_compositor_frame_sink_manager.cc",
-    "renderer_host/offscreen_canvas_compositor_frame_sink_manager.h",
     "renderer_host/offscreen_canvas_provider_impl.cc",
     "renderer_host/offscreen_canvas_provider_impl.h",
     "renderer_host/offscreen_canvas_surface_impl.cc",